Block copolymer of poly(dioxa-amide) and polyamide

ABSTRACT

Semipermeable membranes useful in membrane separation processes are formed from a hydrophilic polyamide which is a block copolymer of nylon and a poly(dioxa-amide) such as poly(4,7-dioxadecamethylene adipamide).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 970,409 filedDec. 18, 1978, which is a continuation-in-part of application Ser. No.769,246, filed Feb. 16, 1977, now U.S. Pat. No. 4,130,602 issued Dec.19, 1978, which in turn is a continuation-in-part of application Ser.No. 577,717 filed Mar. 12, 1975, now abandoned, which in turn is acontinuation-in-part of application Ser. No. 418,524, filed Nov. 14,1973, now abandoned.

BACKGROUND OF THE INVENTION

Numerous membrane separation processes are known wherein a materialhaving different permeabilities to two materials is used to separateand/or concentrate a mixture of the materials. Some of these processesinclude diffusion control, gas separation, dialysis, ultrafiltration,osmosis, reverse osmosis, pervaporation, diasolysis, hemodialysis,piezodialysis and forced flow electrophoresis. These processes are wellknown and a good description is in Encyclopedia of Polymer Science andTechnology Vol. 8, page 620, 1968. They all use a semipermeable membraneas the basic separation device although incorporating other featuressuch as pressure, electrical assistance, etc., to speed or otherwiseaffect the separation. Membranes in these processes must not only havethe desired morphology to achieve the appropriate separation but mustprocess easily, have good mechanical properties and be stable in theirenvironments.

SUMMARY OF THE INVENTION

The present invention relates to the use of a specific polymer as thesemipermeable membrane in membrane separation processes such as thosedescribed above. The polymer is as described in U.S. Pat. No. 4,130,602and is a block copolymer of a nylon and a poly(dioxa-amide). Thecopolymer is hydrophilic because of the introduction of the etherlinkages but much improvement in properties is obtained by the blockcopolymer characteristic as opposed to the ether-containing randomcopolymers of the prior art. The copolymers employed in the presentinvention have several outstanding properties which make them eminentlysuitable for membrane separation processes.

DESCRIPTION OF THE INVENTION

The composition used in the separation process is a block copolymer of aspecified polyamide and a specified poly (dioxa-amide). The polyamideportion of the molecule is a bivalent radical of a nylon which is thegeneric description of condensation polymers of a diamine and a dibasicacid. Preferably it is melt spinnable and has no ether linkages Examplesof melt spinnable polyamides having no ether linkages are as follows:nylon-6, 10 [poly(hexamethylene sebacamide)]; nylon-6 [pentamethylenecarbonamide)]; nylon-6,6 (hexamethylene adipamide); nylon-11[poly(decamethylene carbonamide)]; MXD-6 [poly(meta-xylene adipamide)];PACM-9 [bis(para-aminocyclohexyl)-methane azelamide]; PACM-10[bis(para-aminocyclohexyl)-methane sebacamide]; and PACM-12[bis(para-aminocyclohexyl)methane dodecanoamide]. Others are listed inENCYCLOPEDIA OF POLYMER SCIENCE AND TECHNOLOGY, Vol. 10, SectionPolyamide Fibers, Table 12. Methods for preparing these polyamides arewell known and described in numerous patents and trade journals.

The poly(dioxa-amide) portion of the molecule contains both a doubleoxygen linkage, i.e., --R--O--R--O--R-- and amide linkage, i.e.,--N--C--. The following repeating structural formula depicts thecomposition used in this invention wherein the "y" radical is thepoly(dioxa-amide) portion: ##STR1## wherein R₁, R₂ and R₃ are selectedfrom the group consisting of H, C₁ -C₁₀ alkyls and C₃ -C₁₀ isoalkyls; R₄is selected from the group consisting of C₆ -C₁₂ aryls, C₁ -C₁₀alkylenes and C₃ -C₁₀ isoalkylenes; R₅ is selected from the groupconsisting of C₀ -C₁₀ alkylenes and C₃ -C₁₀ isoalkylenes; and y=4-200and z=4-300. Examples of C₁ -C₁₀ alkyls are methyl, propyl, butyl,pentyl, etc.; examples of the C₃ -C₁₀ isoalkyls are isopropyl, isobutyl,isopentyl and the like. Examples of C₁ -C₁₀ alkylenes are as follows:methylene, dimethylene, trimethylene and the like; examples of C₃ -C₁₀isoalkylenes are as follows: methyltrimethylene, 2-methyltetramethyleneand the like. The molecular weight of the copolymer is about5000-100,000, preferably 10,000-50,000.

The poly(dioxa-amide) portion of the composition can be prepared by thefollowing generalized scheme: ##STR2##

Examples of HOR₄ OH of reaction (1) are as follows: ethylene glycol,propylene glycol and trimethylene glycol. Examples of HOOCR₅ COOH ofreaction (3) are as follows: malonic, succinic, glutaric, adipic,pimelic, suberic, azelaic sebacic, undecanedioicα,α-diethylsuccinic andα-methyl-α-ethyl suberic.

Reaction (1) is often referred to as cyanoethylation, reaction (2) is ahydrogenation, reaction (3) is the reaction between a diacid and diamineresulting in a salt and reaction (4) is often referred to as acondensation polymerization, in which case the repeating unit containsfewer atoms than the monomer, and necessarily, the molecular weight ofthe polymer as formed is less than the sum of the molecular weights ofall the original monomer units which were combined in the reaction toform the polymer chain.

A variation of preparation reactions (1) and (2) is also disclosed inCHEMICAL ABSTRACT 3935K, Vol. 71 (1969) S. African Pat. No. 6,704,646.

Examples of poly(dioxa-amide) polymer that can be prepared in theaforementioned generalized scheme are the following: ##STR3##

The aforementioned y and z both can equal 4 and are preferably at least15. Increasing the value of y tends to increase the permeability offilms made therefrom. Preferred maximum values of y and z are about 175and 185, respectively, more preferred values are about 100 and 130 butvalues of 200 and 300 are operative. Values of y and z are medianvalues.

Films can be made from the copolymer by conventional techniques such assolvent casting with aqueous formic acid or tetrafluoroethanol.Alternatively, melt, i.e., extrusion techniques can be employed.

The polymers of present invention can also contain an antioxidant suchas1,3,5-trimethyl-2,4,6-tris(3,5-ditertiarybutyl-4-hydroxybenzyl)benzene.Small amounts of antioxidant, e.g., 0.5 weight percent, aresatisfactory, however, as little as 0.01 weight percent can be used oras much as 2.0 weight percent also can be satisfactory. Antioxidantsother than the aforementioned one can be used. The antioxidant generallywould be mixed in combination with the two polymers prior to meltblending. Other usual additives for polyamides such as delusterantsand/or light stabilizers can also be incorporated.

EXAMPLE 1

A copolymer is prepared according to the procedure of Run 11, U.S. Pat.No. 4,130,602, except that 275° C. and 45 minutes is used instead of295° C. and 30 minutes. The copolymer has a molecular weight of about30,000 and y and z values of 111 and 167 respectively.

EXAMPLE 2

The polymer prepared in Example 1 is formed into a film about 2 milsthick using a drum two inches in diameter and spinning at about 1725 RPMby virtue of a shaft on the drum attached to an ordinary motor. Thepolymer is dissolved in a solvent of 80% formic acid-20% water and isadded to the spinning drum in a hood at 40° C. As spinning progressesthe film deposits on the inside of the drum and solvent evaporates outthe open end thereof. After several minutes the film is removed from theinner surface of the drum.

A membrane separator consists of a closed Par reactor. Inside thereactor a porous steel plate 11/2 inches in diameter has a hole in thecenter connected to the atmosphere by tubing through a hole in the topof the reactor. The copolymer film is attached to the underside of theplate. Means are provided to supply air pressure to the reactor.

The reactor is charged with a 31/2% NaCl solution to about fill thereactor. Air pressure in the reactor is increased to and maintained at500-800 psi. The salt solution passes through the membrane and iscollected outside the reactor. The effluent shows an NaCl rejection of99% at a flow rate of 20 gal/FT² /day.

We claim:
 1. A block copolymer having a molecular weight of about5000-100,000 and the following repeating structural formula: ##STR4##wherein R₁, R₂ and R₃ are selected from the group consisting of H, C₁-C₁₀ alkyls and C₃ -C₁₀ isoalkyls;R₄ is selected from the groupconsisting of C₁ -C₁₀ alkylenes and C₃ -C₁₀ isoalkylenes; R₅ is selectedfrom the group consisting of C₀ -C₁₀ alkylenes and C₃ -C₁₀isoalkylenes;said melt spinnable polyamide is selected from the groupconsisting of nylon 6, 10; nylon-6; nylon-6,6; nylon-11; MXD-6; PACM-9;PACM-10 and PACM-12; and y=4-200 z=4-200.
 2. Copolymer according toclaim 1 wherein the copolymer is hydrophilic.
 3. Copolymer according toclaim 1 wherein R₁, R₂, and R₃ are H and R₄ is C₁ -C₁₀ alkylene. 4.Copolymer according to claim 3 wherein R₅ is C₁ -C₁₀ alkylene. 5.Copolymer according to claim 3 wherein R₄ is dimethylene.
 6. Copolymeraccording to claim 4 wherein R₅ is the alkylene radical of adipic acid.